Many trucks have a cab which is normally directly over the engine and can be tilted forwardly to expose the engine for repair and adjustment. An hydraulic power system for so tilting the cab comprises an hydraulic piston and cylinder assembly with one end attached to the chassis and the other end attached to the cab. When the assembly is in its retracted position, the cab is in its driving position. The assembly can be fully extended to tilt the cab through an angle of approximately 90.degree. to give full vertical exposure to the engine and componentry therebeneath. However, full 90.degree. tilt is rarely needed, for most engine repair and maintenance work can be done with the cab at an angle of about 45.degree.. During approximately 45.degree. of movement, the weight of the cab exerts a force tending to oppose that movement, and if the hydraulic force were released, the cab would fall freely back to the driving position. Also, if the cab is moved beyond the balanced position, it exerts a pulling force on the piston and the piston rod, tending to accelerate its movement. In other words, when the cab is moved past the balanced position, only hydraulics prevents it from falling freely.
As a precaution against accidental cab fall, for example in the event of an unexpected loss in hydraulic pressure, the trucks require a safety system to prevent free-fall by blocking the hydraulic fluid and locking the cylinder and piston assembly. Also, it is important to be able to lock the cab in any intermediate position. Further, it is important to maintain control and provide for user safety by assuring locking of the cab whenever the cab tilting system is not in active use.
Manual work is required to operate the pump which tilts the cab; so the cab is only raised up to the point needed, thus eliminating unnecessary additional work. Since the fully tilted position is rarely needed, the cab is usually tilted up to about 45.degree.; this sufficiently exposes the engine and componentry for routine maintenance. Controlled return free-fall, or gravity induced lowering of the cab, is desired for between the balanced position (about 45.degree.) and the driving position, since it eliminates the work required to pump the cab back down. Controlled free-fall in the direction between the balanced position and the fully tilted position is not as important as free fall in the return direction, because this range of tilt is so infrequently used.
Two principal approaches have been taken in the prior art attempts at solving the problems of free-fall and safety stopping; only one of the two problems is solved in any one prior system.
One such approach was to prevent free-fall altogether. In a system, such as that described in U.S. Pat. No. 3,472,547 to London, a pressure-controlled safety valve was used to hold the cab in any pivoted position. This system required that the cab be pumped through its entire range of travel from 0.degree. to 90.degree., full tilt and back.
The other approach was to control free-fall, but this approach did not enable the cab to be locked in any intermediate position. Free-fall control could be obtained by a fluid-flow responsive restriction in the hydraulic fluid return line. U.S. Pat. No. 3,761,123, to Neill, et al. discloses a flow controller having a velocity fuse plus a bypass conduit with a fixed restricted orifice or, alternatively, a pressure-compensated flow-control valve. This system did not, however, provide for emergency arrest of the cab's movement, but merely slowed down that movement. Other systems, such as that of U.S. Pat. No. 3,972,557 to Hudston et al. illustrate combinations of check valves and flow restrictors, such as a lock valve in the lower hydraulic fluid inlet line and a unidirectional speed restrictor in the upper inlet line.
An object of this invention to to provide both controlled free-fall from the balanced position back to the normal driving position, and safety protection from an inadvertent loss of hydraulic pressure, both of which are controlled from a common centralized location. In addition, the cab can be purposely locked at any desired degree of tilt.
Another object of this invention is to provide on operator safety switch with "dead-man type" emergency stopping of cab movement, requiring constant manual pressure during controlled free-fall.